Question

Describe the change in hybridisation (if any) of the Al atom in the following reaction.
$AlCl_3+Cl^- \rightarrow AlCl_4^-$

Answer 1

The valence orbital picture of aluminium in the ground state can be represented as:

The orbital picture of aluminium in the excited state can be represented as:

Hence, it undergoes $sp ^2$ hybridization to give a trigonal planar arrangement (in $AlCl_3$).
To form $AlCl_4^ -$ , the empty $3p_z$ orbital also gets involved and the hybridization changes from $sp ^2$ to $sp ^3$ .
As a result, the shape gets changed to tetrahedral.